Printing of an image via an ink-jet system can be performed by a method in which ink droplets are ejected from an ink-jet recording head and adhere onto a targeted recording medium. Such ink-jet system has advantages such as a relatively simple mechanism, the low sale price, and formation of high-definition and high-quality images.
Various types of ink-jet recording systems are known, and an on-demand type recording system, which has become a major leader in recent years, can be categorized as either a so-called piezo type which employs a piezoelectric element or a so-called thermal jet type. Of these, it is known that since the ink-jet recording system employing a piezo method repeatedly applies and reduces pressure to eject ink, minute bubbles tend to form due to cavitation, and the formed bubbles cause blank dots, or dots being out of register due to off-target ejected ink droplets during ejection of ink, to result in graininess which degrades the printed images.
In general, cavitation refers to a physical phenomenon in which a liquid evaporates to form a bubble when the liquid pressure, under specific temperature, becomes lower than the vapor pressure at that temperature. To cope with the adverse formation of bubbles, an employed ink-jet ink is usually subjected to a degassing treatment to reduce as much as possible the quantity of gases contained in the ink-jet ink, which prevents formation of bubbles during ink ejection. With regard to a degassing method, Patent Document 1 discloses a method of degassing via stirring a liquid in a vacuum. However, for dispersion type ink such as a pigment ink, the method exhibited rather accelerated generation of cavitation during degassing treatment since minute bubbles were generated in the liquid during degassing, which bubbles adhered to the surface of pigment particles, while the method degasses without fail solution type inks such as a dye type ink. Patent Document 2 discloses a degassing method by boiling an ink while stirring.
However, the method is not only very time consuming and requires much energy, but also exhibits the problem for pigment type ink such that the boiling treatment exerts significant influence on storage stability of the ink after the boiling treatment, since portions of the dispersed pigment ink coagulate during such boiling. Further, Patent Document 3 discloses a degassing method by feeding an ink into the interior of a gas permeable tube and by reducing air pressure or evacuating the exterior of the tube. The method can effectively degas even a pigment ink unlike either of the aforesaid two methods. However, with regard to degassing of a pigment ink exhibiting high viscosity of not less than 10 mPa·second at 25° C., applying high pressure to the ink was required, since a significant pressure loss during degassing arises when an ink is fed through a minute hollow fiber (namely a tube) having an inside diameter of several tens of microns. In this case, a high pressure applied to a pigment ink before and after the degassing treatment caused mutual coagulation among pigments, to result in exerting a significant adverse influence upon storage stability of such ink. In an extreme case, an ink immediately after exiting the hollow fiber module sometimes exhibits separation. Further, such a hollow fiber module that withstands the above-described high pressure caused problems leading to an oversized module and an increase in cost.
One possibility to decrease pressure loss during degassing might be to increase the inside diameter of the hollow fiber, but the specific surface area per volume of a module became small as the inside diameter increased, to result in a problem to have lowered production efficiency due to a significant decrease in degassing efficiency.
As described above, with regard to a pigment ink exhibiting high viscosity of not less than 10 mPa·second and not more than 50 mPa·second at 25° C., the conventional degassing methods exhibiting many problems, and specifically, large-scale production of an ink exhibited a major problem. Further, an ink-jet printer for industrial use requires incomparably higher ejection reliability than a conventional home printer. An industrial ink-jet printer uses many nozzles and high ink droplet ejection frequency to enhance productivity. Under such conditions, even a small fluctuation of ejection rate of ink-jet droplets or occurrence of a nozzle which prevents ejection of ink droplets, which leads to image defects, is not acceptable. A means to counter cavitation merely by feeding an ink degassed in advance into the ink flow channels has limits of reliability to prevent the aforesaid cavitation. Therefore, required is a recording system or a printer, in which an ink is degassed during flowing in an ink flow channels before ejection. In response to such problems, technologies of an ink-jet recording apparatus are disclosed (as described, for example, in Patent Documents 4 and 5), in which degassing is carried out in a printer by arranging an internal refluxing type hollow fiber degassing module between the ink tank and the printhead. However, even in this case, a high viscosity ink having viscosity of not less than 10 mPa·second causes a significant pressure loss in a hollow fiber degassing module during degassing, so that a large solution feeding pump is required to apply high enough pressure to the ink, resulting in problems that the printer becomes oversized and the sale price rises.
On the other hand, since an ink-jet printer features a system of ejecting minute liquid droplets from minute nozzles, to achieve a precise ink ejection, the so-called ink-jet head maintenance such as periodical discharge of an ink, wiping the nozzles of a nozzle plate, and discharge of all inks remaining in the ink channels is required to enhance reliability.
Recently, with the ongoing progress of high-speed ink-jet, the number of nozzles incorporated in one unit of a printer has been dramatically increased. Further, so-called line head printers featuring the nozzle density corresponding to a printing resolution over the total printing width has also been introduced. For example, the number of nozzles required for printing width of 600 dpi (dpi refers to a number of dots per an inch, that is, per 2.54 cm) for the long side (297 mm) of A4 size printing medium reaches 7,016 per color. With the increase in the number of nozzles, the amount of ink consumed during maintenance can no longer be ignored. Further, since such printers are mostly used in industrial applications, a reduction of printing costs is strongly demanded, and thereby, a reduction in the amount of wasted ink has been demanded.
A method is disclosed (refer to Patent Document 6) in which any ink discharged into a nozzle cap during head maintenance is recirculated to the head and is used for ejection.
Any ink discharged from a nozzle and exposed to air, as described in above Patent Document 6, is rapidly charged with ambient air. When such an ink is ejected, the above cavitation is easily generated to result in marked degraded reliability of ink ejection.
A technology is also disclosed (refer to Patent Document 7) whereby an ink is reliably ejected due to removal of bubbles from an ink channels by sufficient degassing of the ink employing a printer having an ink refluxing channels upstream of the ink-jet head, and a degassing apparatus along the circulation channels. However, during head maintenance, the ink chamber within the head is required to remove all air bubbles via ink discharge through a nozzle, resulting in the problem of any ink consumed during such maintenance to be disposed of.    Patent Document 1: Unexamined Japanese Patent Application Publication No. (hereinafter also referred to as JP-A) 6-287494    Patent Document 2: JP-A 9-59549    Patent Document 3: JP-A 5-17712    Patent Document 4: JP-A 11-42771    Patent Document 5: JP-A 11-48491    Patent Document 6: JP-A 5-330073    Patent Document 7: JP-A 11-42795